The postnatal development of the transitional region (TR) i.e. the proximal free part of a spinal rootlet that contains both PNS and CNS tissue, was studied light-microscopically in semi-thin sections and after histochemical staining according to the Marchi and OTAN methods for the demonstration of degenerating myelin and according to the Gomori method for the demonstration of acid phosphatase activity. In the newborn kitten the PNS tissue extended well up to the spinal cord surface and the rootlets lacked a transitional region. The CNS tissue entered the root during the second postnatal week, and a trasitional region was fully established at the beginning of the second month. The degree of myelination in the group of large fibres differed on the two sides of the PNS-CNS borderline: well myelinated PNS fibres were transformed into poorly myelinated or apparently unmyelinated CNS-fibres. PNS and CNS myelin sheaths of large fibres appeared to be of equal thickness in the 4 week old kitten. During the first postnatal month large amounts of Marchi positive material and a high acid phosphatase activity occurred in complex paranodes and very short internodes in the PNS compartment just distally to the PNS-CNS borderline. In the adult cat Marchi positive bodies were numerous in the CNS compartment just proximally to the PNS-CNS borderline. The results are discussed against previous studies on focal demyelination as found during the normal development of the feline peripheral nervous system.